1. Field of the Invention
This invention relates in general to wireless digital communications. In particular, the invention relates to an improved frame structure for use in time division multiple access frequency hopping radio communications.
2. Background Art
Devices incorporating wireless communication techniques are becoming increasingly prevalent in modern society. An inevitable result of this trend is that the frequency spectrums over which these communications take place are becoming more crowded and prone to interference. At the same time, consumers are also greatly concerned about issues of privacy and security of communications. Consequently, system engineers designing a variety of wireless communications systems, including cellular and cordless telephones, often turn to digital spread spectrum signaling methods to achieve better voice quality, greater security, and more efficient bandwidth utilization than can be achieved with other signaling methods, such as conventional amplitude or frequency modulation.
One popular spread spectrum signaling technique is frequency-hopping spread spectrum (“FHSS”) technology. A FHSS transceiver operates by rapidly changing the frequency at which it receives and transmits signals in a known pattern, called the hop sequence or hop pattern. By using their own unique hop sequences, multiple systems can communicate simultaneously over a common frequency bandwidth. FHSS offers better voice quality and improved reliability compared to other solutions in noisy environments because only a short segment of data is transmitted on any given frequency channel. Therefore, if a channel is noisy or otherwise prone to interference, that data segment can simply be discarded. When the number of bad channels in the hop sequence is relatively low, the resulting degradation in data throughput is relatively minor.
In many FHSS applications, such as cordless telephony, a two-way communications link is required. One technique for implementing such a two-way communications link is called Time Division Duplexing (“TDD”). In a TDD system, each of two communications devices joined by a communications link can both transmit and receive data in each frame. A typical TDD FHSS data frame structure, as might be implemented by a cordless telephone base station communicating with a single cordless telephone handset, is illustrated in FIG. 1. Each frame 100 occupies one frequency hop in the hop sequence with the successive frame operating at the next frequency in the hop sequence. The frame begins with guard band 110. The guard band provides a time period during which the base station transmitter can couple to the base station antennae and the transmitter's phase locked loop (PLL) can settle on the required carrier frequency for transmission. Subsequent to guard band 110, the base station transmitter transmits any desired data signals during time slot 120. Another guard band 130 is provided to allow the base station receiver tuning PLL to settle on the frequency required for reception as the base station sets up to receive data, and then the base station receiver monitors the channel during time slot 140. Finally, guard band 150 provides a time period during which any required switching occurs, and the transceiver tunes to the next frequency in the hop sequence as it enters the subsequent frame. Optionally, received signal strength indicator (RSSI) period 160 can be utilized by the receiver section to monitor the level of signal received on the channel during a period in which desired communication signaling does not occur, thereby providing an indication of the level of undesired noise on the particular channel.
Another aspect of many FHSS systems that is particularly advantageous is the ability to circumvent sources of interference at static frequencies by dynamically changing the frequency channels in the hop sequence, substituting a new frequency channel for a channel that is identified as having excessive noise. This process is commonly referred to as frequency adaptation, or dynamic channel allocation. Numerous methods of monitoring channel performance and determining when a channel should be removed from the hop sequence are known in the art.
Finally, many wireless systems also include time domain multiple access (TDMA) features so that multiple devices can communicate over a given wireless communications link. TDMA involves the division of a data frame into multiple time slots, whereby each device can communicate during its own time slot. One technique for implementing TDMA in a FHSS data frame is called a slow-hopping technique, and is illustrated in the graph of FIG. 2. This slow-hopping technique involves the transmission of a complete frame of data on each frequency in the hopping sequence. In the TDMA example of FIG. 2, each frame is subdivided into four bidirectional time slots that can be utilized by different communications devices. Each time slot 1 through 4 further includes a downlink period T and a corresponding uplink period R. For example, a wireless PBX telephone system can be implemented using the TDMA frame of FIG. 2 in which each of four handsets receives data from the base station during periods T1, T2, T3 and T4, respectively. Each of the four handsets then transmits data to the base station during periods R1, R2, R3 and R4, respectively. Therefore, multiple devices are capable of communicating over the single data frame operating on a single frequency in time.
However, slow-hopping systems such as that illustrated in FIG. 2 may inhibit the use of frequency adaptation techniques. This is because when a new handset is introduced into the system, or when an existing handset loses contact with the base, that handset must be resynchronized, the handset must “listen” to the base unit using the same hopping pattern, and thus the same frequencies, as the base unit to acquire synchronization. If the hopping pattern used by the base unit continually changes in accordance with a frequency adaptation technique, then a new handset attempting to wirelessly synchronize with the base unit cannot know the current hopping sequence and will therefore be unable to synchronize with the system. While in a single handset system the base unit can simply revert back to a default hopping sequence known to all handsets as soon as it loses contact with its handset, in a multiple access system the base unit may be actively communicating with other handsets using the optimized and perpetually adapting hop sequence, and may therefore be unable to promptly revert to a default sequence. If a multiple access system did switch each active communications link back to a predetermined non-adaptive hop sequence whenever the system is open synchronization of a new handset, then the handsets that are already synchronized and communicating cannot take advantage of adaptive interference avoidance techniques. Thus, conventional slow-hopping frame structures are not well-suited to TDMA FHSS systems that implement frequency adaptation techniques.
Another common TDMA frequency hopping scheme is one in which the carrier frequency hops, or changes, between each handset time slot. This technique can be referred to as a fast-hopping system, and is illustrated in FIG. 3. This type of fast-hopping technique is implemented in the WDCT cordless telephone protocol, which was adapted for the North American market and based, at least in part, upon the European DECT standard. Since the frequency of each time slot in a fast hopping system can be controlled independently, a fast hopping system can maintain a subset of time slots at fixed frequencies to facilitate synchronization of new communications deivces.
However, because such fast-hopping techniques require that the communication device transceivers change tuning frequencies between each time slot, rather than once per frame as in a slow-hopping system, a separate guard band is required between each time slot to allow the receiver and transmitter tuning frequencies to stabilize between frequency hops before any data transmission or reception occurs. As the number of time slots per data frame is increased, the proportion of each data frame consumed waiting for frequency stabilization during guard band periods increases accordingly. Thus, while the illustrated fast-hopping system allows for the use of independent hop sequences on each time slot, the data bandwidth—and accordingly the number of time slots that can be supported for a given frame length—is substantially decreased.
These and other desirable characteristics of the present invention will become apparent in view of the present specification, including claims, and drawings.